1. Field of the Invention
The present invention relates to a variable speed switch for adjusting a rotational speed of an electric power tool, which includes a resistive plate and a brush that slides in a given direction with respect to the resistive plate by a pull operation of a trigger, and in which resistance of the resistive plate varies according to the slide of the brush with respect to the resistive plate.
2. Description of the Related Art
An example of this kind of variable speed switches is shown in FIG. 7(A).
The variable speed switch 100 includes a substrate 101, a brush 107 that slides in an arrowed direction (horizontal direction) with respect to the substrate 101, and a trigger (not shown) to which the brush 107 is attached, A conductor part 103 and a printed circuit resistor 105 are formed on the surface of the substrate 101 extending horizontally. A far left side and a far right side of the printed circuit resistor 105 are connected to terminals T0 and T1, respectively, and the conductor part 103 is connected to a terminal T2. Further, the far left part and the far right part of the brush 107 can be slidably contacted to the conductor part 103 and the printed circuit resistor 105, respectively.
When the trigger is pulled in the variable speed switch 100, the brush 107 slides from the far left side in a rightward direction according to pulling amount of the trigger and the resistance between the terminals T1 and T2 (the resistance of the variable speed switch 100) decreases gradually. FIG. 7(A) shows a state in which the trigger is pulled to the maximum position. FIG. 7(B) shows an equivalent circuit of the variable speed switch 100.
FIG. 8 is a schematic showing an electric circuit of the electric power tool equipped with the above-described variable speed switch 100. A reference voltage from a controller 102 is applied between the terminals T0 and T1 of the printed circuit resistor 105 in the variable speed switch 100. Consequently, a voltage signal outputted from the terminal T2 drops as the resistance of the variable speed switch 100 decreases. A controller 102 of the electric power tool controls a switching element FET such that a current flowing through a motor M increases according to a drop of the voltage signal outputted from the variable speed switch 100. Thus, a rotational speed of the electric power tool increases by the pull operation of the trigger in the variable speed switch 100.
However, because the above-described variable speed switch 100 is constructed such that resistance varies by sliding the brush 107 with respect to the substrate 101, a contact between the brush 107 and the substrate 101 becomes unstable when the brush 107 etc. becomes worn with time, and the resistance varies unstably when the trigger is pulled as illustrated in FIG. 7(C). Especially, when the resistance fluctuates unstably at the time of the trigger being pulled to the maximum position (refer to the arrow R of FIG. 7(C)), the maximum output (the maximum rotational speed) of the electric power tool fluctuates (refer to the arrow N of FIG. 7(D)), which may deteriorate work efficiency.
To improve this defect, a configuration of electric circuit is used in which a mechanical contact 103 is provided in parallel with the switching element FET, and the mechanical contact 103 is switched ON when the trigger is pulled to the maximum position. Consequently, the maximum output of the electric power tool may not drop and deterioration in the work efficiency may be reduced. However, in this modification, even if the controller 102 controls motor current by sensing a signal showing a drop of a battery voltage 104, the controller 102 cannot prevent over-discharge of the battery 104. This is because motor current flows regardless of a discharge control signal from the controller 102 when the trigger is pulled to the maximum position.
In a variable speed switch described in Japanese Laid-Open Patent Publication No. 7-220563, a rotational speed of the electric tool can be adjusted according to a pressure force applied to a pressure sensor attached to the trigger, which may solve the above described problem.
However, a configuration in which the pressure sensor is attached to the trigger of the variable speed switch may cause a large cost increase.
Thus, there is a need in the art to obtain a rotational stability at low cost and to prevent over-discharge of the battery when an electric power tool is operated at a full speed, without modifying a configuration for sliding a brush with respect to the resistive plate.